Download Species Evenness Where

Species Abundance and Diversity
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Communities
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Habitat Selection
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Filling the available niches
and keeping out
competitors
Exception humans
 All habitats
 Left Africa and adapted
the environment rather
than adapted to the
environment
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Introduction
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•
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Community: Association of interacting
species inhabiting some defined area.
 Community Structure includes attributes
such as number of species, relative
species abundance, and species diversity.
Guild: Group of organisms that all make their
living in the same fashion (insectivorous).
 Seed eating animals in the desert.
Life Form: Combination of structure and
growth dynamics.
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Communities, Guilds and
Competition
•
Organisms in a given
area make up a
community


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•
Spatial
May be subset of whole
community with some
linking characteristics
Organisms competing
for the same resource
make up a guild
In both cases, the effects
can be positive, negative
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or neutral
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•
Species numbers in
a community can
vary from very
abundant and
dominant to rare by
essential to the
community
Species diversity

Species richness


Species evenness

•
How many
individuals
Dominants


•
How many species
Big trees in a forest
Removal changes
the character of the
system
Keystone


Predators
Removal changes
limits of the system
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
Food Web
 How
resources
move
about in
terms of
predation,
competitio
n,
parasitism
,
mutualism
, etc
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Communities have a defined physical structure than is relatively
stable
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Vertical as well as hroizontal
structure
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One community can have more
than one zone
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•
•
•
•
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Defining the edges
of communities can
be both complex
and difficult
Statistical
Scale dependent
Changes with time
over the year
Changes with time
over longer periods

Primary

New environment


Volcanic island
Secondary

After disturbance


Human
None human
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Succession works for the whole
system
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Species Abundance
•
There are regularities in the relative
abundance of species in communities
•
Concept of distribution of commonness and
rarity.
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Lognormal Distribution
•
Preston graphed abundance of species in
collections as frequency distributions.
 Lognormal Distributions
 Bell-shaped curves.
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Lognormal Distribution
•
In most lognormal
distributions, only portion
of bell-shaped curve is
apparent.
 Sample size has
large effect.
 Significant effort
to capture rare
species.
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Species Diversity
•
Two factors define species diversity:
 Species Richness
 Number of species in the community.
 Species Evenness
 Relative abundance of species.
EstimateS SOFTWARE
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Species diversity is a measure of both the species
richness and species evenness of a community
•
Species Richness

•
Species Evenness

•
the relative abundance of individuals within a species
Species Dominance

•
the number of species in a community Species
one or two species greatly outnumbering others (contrasts with
evenness)
When comparing two communities, greater richness often
equals greater diversity. However, in communities with
equal numbers of richness, evenness or dominance must
be considered.
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Species Diversity
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Measuring Diversity:
Species Richness (R)
The species richness is based solely on the number of
species found in the given area and does not reflect
the relative dominance of species.
Rs
Where:
s = the number of species
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Measuring Diversity:
Shannon-Wiener Index (H)
This index is determined by both the number of species and the even
distribution of individuals among those species (relative dominance). It
indicates the degree of uncertainty of predicting the species of a given
individual picked at random from the community. In other words, if the
diversity is high, you have a poor chance of correctly predicting the
species of the next individual picked at random.
s
H   pi  ln  pi 
i 1
Where: pi (relative abundance) = ni/N
ni = number of individuals in species i
N = total number of individuals in all species
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Quantitative Index of Species Diversity
•
Shannon Wiener Index:
s
H’ = -∑pi logepi
i=l
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•
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•
H’
pi
25/100
loge
s
= Value of SW diversity index.
= Proportion of the ith species.
= Natural logarithm of pi.
= Number of species in community. 23
Measuring Diversity:
Species Evenness
Evenness (E) is a measure of how similar the abundance of different
species are. When there are similar proportions of all species then
evenness is one, but when the abundance are very dissimilar (some
rare and some common species) then the value increases.
H
E
ln R 
Where: H is the Shannon-Wiener index
R is species richness
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Measuring Diversity:
Species Dominace
Dominance represents the most commonly occurring
species in a community as a percentage of the total
community
N max
N
Where: Nmax is the number of individuals in the most
commonly occurring species
N = total number of individuals in all species
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Rank Abundance Curves
•
Can also portray relative abundance and
species diversity within a community by
plotting relative abundance of species
against their rank in abundance.
 Greater evenness indicated by lower
slope.
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Rank Abundance Curves
Caddisflies
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Environmental Complexity
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•
In general, species diversity increases with
environmental complexity or heterogeneity.
MacArthur found warbler diversity increased
as vegetation stature increased.
 Measured environmental complexity as
foliage height.
Many studies have shown positive
relationship between environmental
complexity and species diversity.
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Environmental Complexity
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Disturbance and Diversity
•
•
Disturbance - departure from “average
conditions.”
Sousa defined disturbance:
 Discrete, punctuated, killing,
displacement, or damaging of one or more
individuals that directly or indirectly
creates an opportunity for new individuals
to be established.
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Disturbance and Diversity

Any relatively discrete event in time that
disrupts ecosystem, community, or
population structure and changes
resources, substrate availability, or the
physical environment.

Two major characteristics:
 Frequency
 Intensity
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Intermediate Disturbance Hypothesis
•
Connell proposed disturbance is a prevalent
feature that significantly influences
community diversity.
 Proposed both high and low levels of
disturbance would reduce diversity.
 Intermediate levels promote higher
diversity.
 Sufficient time between disturbances
allows wide variety of species to
colonize, but not long enough to allow
competitive exclusion.
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